You can use the same secret key that is already included in the repo.
You can also create a 256 bit key from any online website or command line. I will leave how to do this as a exercise of your own accord.
You can give whatever issuer name you want, or use already included value
same with audience
But, you must ensure, all the projects that consume the auth service, use the exact same values and the values are updated in the corresponding appserttings.json file.